Coupling with strong lens and weak lens on flexure

ABSTRACT

Modern optoelectronic components have very tight coupling tolerances. Small misalignments of a strong lens that occur during the alignment and bonding process seriously degrade optical coupling. A weak lens is actively mounted using a flexure to correct misalignments of the strong lens. Since the weak lens does not exert as much steering force on a beam for a similar movement as the strong lens, misalignments that may occur during weak lens positioning and bonding do not appreciably degrade coupling.

FIELD OF THE INVENTION

[0001] Embodiments of the invention relate to optoelectronic assemblyand, more particularly, to methods and apparatuses for facilitatingprecision alignment between various optoelectronic components.

BACKGROUND INFORMATION

[0002] One of the major challenges in the optoelectronic assemblyprocess is to couple light from one chip to another chip or waveguidewhile maintaining tight tolerances. In brief, the alignment process cangenerally be summarized in just a couple of steps.

[0003] First, the two components are aligned. Tight tolerances arerequired. For example, tolerances of less than 50 nm of precision arenot uncommon between the components. Second, the components must bebonded or otherwise secured to a surface while being careful to keep thealignment.

[0004] Finally, the assembly needs to be reliable. That is, the finishedassembly including the bonding must be stable under temperature cycling,aging, shock, vibration, and any other condition that the assembly mayreasonably be expected to encounter. To further complicate matters, mostassemblies include more than just two components which must all bealigned. Each additional component further adds to the challenge. It isvery difficult to hold the alignment while making the bond. Often someshift or movement occurs between the components which, if greater thanthe minimum tolerances dictate, may render the component unworkable orat least seriously degrade performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a block diagram of a lens coupling assembly coupling twooptical chips; namely, a laser and a modulator;

[0006]FIG. 2 is a block diagram of the lens coupling assemblyillustrating the beam shift due to strong lens misalignment;

[0007]FIG. 3 is a block diagram of the lens coupling assemblyillustrating a using a weak lens on a flexure to compensate for stronglens misalignment;

[0008]FIG. 4 is a graph comparing the coupling efficiency variation dueto movement of strong and weak lenses;

[0009]FIG. 5 is a plan view of a weak lens mounted to a two leg (bipod)flexure;

[0010]FIG. 6 is a plan view of a four legged flexure used to mountlarger components;

[0011]FIG. 7 is a block diagram showing a weak lens and a strong lensarrangement for a pluggable module; and

[0012]FIG. 8 is a block diagram-of a weak lens and strong lens used inan external cavity laser.

DETAILED DESCRIPTION

[0013] One embodiment of the present invention uses a combination ofstrong lenses and weak lenses to meet tight alignment tolerances. Astrong lens is a lens which exhibits a large steering influence on abeam while a weak lens is one that exhibits a smaller steering influenceon a beam. A strong lens is any small focal length lens and may include,for example, molded asphere lenses, graded index (GRIN) lenses, or balllenses having a small focal length. By contrast, a weak lens is anylarge focal length lens and may also include molded asphere lenses, GRINlenses or ball lenses configured to have a larger focal length. Ofcourse many types and manufacturers of strong and weak lenses arecommercially available with would be suitable.

[0014] Referring to FIG. 1, there is shown an assembly for aligning alaser on a chip 10 with a modulator on a chip 12. From left to right,there is shown a laser chip 10. A point 14 is shown to illustrate thebeam emerging from the laser chip 10. Next is a strong lens 16, such as,for example, a molded asphere, an isolator 18, and finally a weak lens20, such as, for example, a GRIN lens. The point at which the beam 22couples with the modulator chip 12 is also illustrated simply as a point24. The isolator 18 is optional but is useful to ensure that the lightflows in only one direction and avoids feedback. The goal of course isto align the various parts so that the beam 22 optically aligns and issuccessfully coupled between the laser chip 10 and the modulator chip12. Of course, while the coupling shown is between a laser and amodulator, it will be appreciated by those skilled in the art thatapplication may be found to couple other optoelectronic components justas easily.

[0015] Referring to FIG. 2, the strong lens 16 is shown shifted 10 μm inthe vertical direction. An unintentional alignment shift such as thismay occur during the bonding process. As shown, a relatively small shiftin the strong lens 16 results in a much larger shift in the finalposition of the beam. Here, the 10 μm shift of the strong lens 16 shiftsthe beam by 40 μm from point 24 to 24′. Since the strong lens 16 exertsa large steering force on the beam 22. Thus, when a strong lens is used,it must be precisely aligned since the slightest misalignment canseriously degrade coupling efficiency.

[0016] As shown in FIG. 3, a weak lens 20 may be used to correctmisalignments of the strong lens 16. Since the weak lens 20 cannot exertas much steering force on the beam 22 small movements or repositioningof the weak lens 20 has little effect on the beam 22. In order tocompensate for the misalignment of the strong lens 16, the weak lens 20must be moved considerably further in a direction to compensate for themisalignment of the strong lens 18. Here, a 10 μm misalignment of thestrong lens 18 requires, for example, a 130 μm adjustment of the weaklens 20 in the opposite direction to bring the beam focus 24 back to itoptimal coupling position.

[0017]FIG. 4 is a graph plotting lens position of example strong andweak lenses as a function of laser to modulator coupling efficiency withthe origin (at 0 μm) being the optimum alignment position. As shown, amisalignment of the strong lens just a few μm from the origin can resultin complete coupling failure. On the other hand, a much greater movementof the weak lens 20 would be required to have the same effect. Herein,this relationship between strong and weak lenses is exploited tofacilitate precision alignment of components.

[0018] Referring to FIG. 5, one embodiment of the invention uses aflexible mount, hereinafter referred to as a flexure 50, on which tomount at least the weak lens 20. The flexure 50 allows for some amountof vertical adjustment. In one embodiment, the flexure 50 is made ofthin spring steel that has been etched or stamped, then bent in a press.The flexure 50 may comprise two or more legs 52 which rest on thesubstrate surface or positioning floor. A two legged flexure may bereferred to as a bipod flexure. In one embodiment, the legs are joinedby a bridge 54 that supports the lens 20 or other optical components.Apertures between the legs 52 and the bridge 54 may be provided toincrease the elasticity of the flexure 50. When the bridge 54 istranslated in the y direction, opposite legs 52 give elastically inopposite x directions thus lowering the bridge and likewise changing they-position of the lens 20. Various flexure designs are described in U.S.Pat. Nos. 6,207,950 and 6,227,724.

[0019] The flexure 50 may be designed so that in its natural ornon-flexed state, the optical axis of the optical component, such aslens 20, attached to the bridge rests slightly above the optical planeof the package. Final adjustment of the height is obtained by applyingpressure to the flexure 50, thereby lowering the bridge 34 height.Dragging the flexure 50 in the plane parallel to the plane of thesubstrate (i.e., in the x-direction) may be used to correct the lateralposition. When adequate alignment is reached, the legs 52 arepermanently attached to the floor or package substrate below. Theattachment may be by, for example, laser welding, soldering, or adhesivebonding. Slots 53 may be provided to facilitate attachment.

[0020] As shown in FIG. 6, in another refinement of the flexure design,the flexure 50 has more than two legs 52. In this case, the first pairof legs 52 is attached to the frame after coarse optical alignment. Theflexure is then finely realigned, using the residual flexibility leftafter the two first legs 52 are attached. When the optimum position isreached, the remaining legs 52′ are attached. This flexure 50′ would beused and a mount for physically larger components requiring alignment.

[0021] Referring now to FIG. 7, an embodiment of the present inventioncomprises placing the weak lens 20 on a flexure. As shown, a pluralityof optical components are placed on a substrate 70. A laser 10 ismounted to the substrate 70 or may be mounted on a sub-mount 72 which inturn is mounted to the substrate 70. A beam 22 originates from a laser10 illustrated as a point source 14 to be coupled to a modulator 12 orother optical component. As previously explained, the beam 22 passesthrough a strong lens 16, and a weak lens 20 in its path. Other opticalcomponents may also be in the path such as an isolator 18 to keep thelight from reflecting backwards. An optical fiber 74 is also showncoupled to the assembly by, for example, a ferrule 76

[0022] During the manufacturing process, all of the components arealigned substantially as described above. That is, two components areoptically aligned on the substrate 70 and then bonded with a laser weld78 utilizing slot 53 or otherwise secured to the substrate 70 whilebeing careful to keep the alignment. Of course, while a laser weld 78 isshown any suitable bonding method may be used such as adhesive bondingor soldering.

[0023] However, no matter the level of care used, some misalignmentoften occurs. Thus, the weak lens 20 is provided on a flexure 50 forfine tuning the alignment. Small misalignments due to the strong lenscan be corrected by relatively larger movements of the weak lenssteering the beam in a direction opposite to the strong lens 16. Oncethe strong lens 16 is aligned and in position, it is securely bonded inplace. Thereafter, the flexure 50 on which the weak lens 20 is mountedis positioned bonded in place when the weak lens is optically aligned ina position that corrects any misalignment of the strong lens and causesthe end point of the beam 24 to be optically coupled with the nextcomponent in the path; here a modulator 12.

[0024] Of course, the flexure 50 securing the weak lens 20 may alsosuffer some misalignment when bonded in place. However, since smallmisalignments in the weak lens 20 have relatively little effect on thefinal position of the beam 24 the optically coupling is improvedoverall. Optionally, the strong lens 16 can also be positioned andbonded in place with the aid of a flexure 50. To the extent the flexure50 facilitates positioning any or all of the optical components may besecured with a flexure 50.

[0025] Passive placement refers to pick and place of components usingmachine vision (i.e., using a machine to pick and place componentsguided by a camera looking at the placement). Active alignment refers topowering up the laser and aligning the components while monitoring thecoupling.

[0026] Ideally, the strong lenses 50, or more generally stronger opticalcomponents, are placed passively and the weak lens 20, or lenses, areplaced actively using the flexure 50. This reduces the number of activealignment elements and decreases the overall module assembly cycle time.The stronger optics will be more stable then if placed and bondedactively, as the bonds will not be stressed.

[0027]FIG. 8 shows another embodiment of the present invention using theflexure 50 using a weak lens 20 on a flexure 50 to tune the opticalcoupling in an external cavity laser. A laser gain chip 80 produces abeam from a back facet 82. The beam proceeds through a strong lens 84, aweak lens 86 and a plurality of filters 88. The beam is then reflectedback with a mirror or reflective grating 90 back through the filters 88,weak lens 86, and strong lens 84, to produce a beam out of the frontfacet 92 of the gain chip 80. As above, once the strong lens 84 isbonded in place, the weak lens 86 is secured to the substrate 94 with aflexure 96. The flexure 96 facilitates placement of the weak lens 86such that misalignments of the other optical components, andparticularly the strong lens 84, can be corrected. Optionally, thestrong lens 84 or any of the other components can also be mounted to thesubstrate 94 with a bipod flexure 98 as shown. With this arrangement,precise optical coupling can be obtained greatly increasingmanufacturing yield and chip performance.

[0028] Embodiments of the present invention are specifically illustratedand/or described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

What is claimed is:
 1. An optical device, comprising: a strong lensmounted to a surface; a weak lens; and a flexible mount to activelyattach said weak lens to said surface, said weak lens mounted in aposition to compensate for misalignments of said strong lens.
 2. Theoptical device as recited in claim 1 wherein said weak lens steers abeam to a lesser extent than said strong lens.
 3. The optical device asrecited in claim 1 positioned to optically couple a beam between twocomponents.
 4. The optical device as recited in claim 3 wherein said twocomponents comprise a laser and a modulator.
 5. The optical device asrecited in claim 1 wherein said optical device comprises an externalcavity laser.
 6. The optical device as recited in claim 1 wherein saidflexible mount comprises a flexure.
 7. The optical device as recited inclaim 6 wherein said flexure comprises: at least one pair of legs madeof a flexible material; and a bridge connecting said legs, wherein saidweak lens is mounted to a top of said bridge.
 8. The optical device asrecited in claim 1 wherein said strong lens comprises a moldedaspherical lens and said weak lens comprises a graded index (GRIN) lens.9. An optical module, comprising: a laser to produce a beam along anoptical axis; a strong lens substantially aligned to steer said beamalong said optical axis; a weak lens to steer said beam; a flexure toactively mount said weak lens in a position to steer said beam alongsaid optical axis to compensate for misalignments of said strong lens;and a ferrule connection aligned to receive the beam from said weaklens.
 10. The optical module as recited in claim 9 wherein said stronglens comprises a molded aspherical lens and said weak lens comprises agraded index (GRIN) lens.
 11. The optical module as recited in claim 10wherein said flexure comprises: at least one pair of legs made of aflexible material; and a bridge connecting said legs, wherein said weaklens is mounted to a top of said bridge.
 12. The optical module asrecited in claim 11, further comprising: a second flexure to mount saidstrong lens.
 13. The optical module as recited in claim 10 furthercomprising: an isolator module mounted along said optical axis betweensaid strong lens and said weak lens.
 14. The optical module as recitedin claim 13 wherein said isolator module is mounted to said secondflexure with said strong lens.
 15. A method for correcting an opticalmisalignment between optical devices, comprising: passively positioninga first lens to steer a beam substantially along an optical axis,bonding said first lens to said surface; actively aligning a secondlens, weaker than said first lens, in a position to steer said beamalong said optical axis to compensate for misalignment of said firstlens.
 16. The method for correcting an optical misalignment betweenoptical devices as recited in claim 15, further comprising: pushing onan elastic mount carrying said weak lens to adjust a y position of theweak lens; and dragging said elastic mount in a plane parallel with saidoptical axis to change an x position of said weak lens.
 17. The methodfor correcting an optical misalignment between optical devices asrecited in claim 15, further comprising: placing a laser in front of aninput end of said strong lens; and placing a modulator in back of anoutput end of said weak lens.
 18. The method for correcting an opticalmisalignment between optical devices as recited in claim 15, furthercomprising: placing a laser before an input end of said strong lens; andplacing one of a mirror and grating after an output end of said weaklens.
 19. The method for correcting an optical misalignment betweenoptical devices as recited in claim 15, further comprising: using one ofadhesive, laser bonding, and solder to secure said elastic mount inplace.
 20. The method for correcting an optical misalignment betweenoptical devices as recited in claim 15, further comprising: positioningan isolator module between said strong lens and said weak lens.